1. Field of the Invention
This invention relates to a production method for a grain oriented electrical steel sheet, having excellent magnetic properties which can be used as the core of transformer, etc.
2. Description of the Related Art
Grain-oriented electrical steel sheet has been primarily used as the core material for transformers and other electrical appliances, and must have excellent magnetic properties such as excitation characteristics, iron loss, and so forth. A flux density B.sub.8 at a magnetic field intensity of 800 A/m is generally used as a numerical value representing the excitation characteristics. The iron loss W.sub.17/50 per kg when the steel sheet is magnetized to 1.7 Tesla (T) at a frequency of 50 Hz is used as a numerical value representing the iron loss. The magnetic flux density has the largest influence on the iron loss. Generally speaking, the higher the magnetic flux density, the better the iron loss. Generally, when the magnetic flux density is increased, the secondary recrystallization grain tends to be coarse, resulting in the inferior iron loss in some case. To cope with this problem, the iron loss can be improved by magnetic domain control irrespective of the size of the secondary recrystallization grains.
This known grain oriented electrical steel sheet is produced by causing secondary recrystallization in a final finish annealing step and developing a so-called "goss texture" having a {110} axis in the direction parallel to the steel sheet surface and a &lt;001&gt; axis in the rolling direction. To obtain particularly excellent magnetic properties, &lt;001&gt; as an easy axis of magnetization must be aligned highly accurately with the rolling direction.
The production technology for such a grain oriented electrical sheet with a high magnetization flux density is typically disclosed in Japanese Examined Patent Publication (Kokoku) Nos. 40-15644 and 51-13469. The former uses MnS and AlN as the main inhibitors while the latter uses MnS, MnSe, Sb, etc. In accordance with the present state of the art, therefore, the sizes and shapes of the precipitates functioning as these inhibitors and their dispersion state must be appropriately controlled. As to MnS, a method is employed at present which produces a complete solid solution of MnS at the time of heating a slab before hot rolling, and allows it to precipitate at the time of hot rolling. To produce a complete solid solution of MnS, in an amount necessary for secondary recrystallization, a temperature of about 1,400.degree. C. is necessary.
This temperature is at least about 200.degree. C. higher than the slab heating temperature for an ordinary steel, and this high-temperature slab heat-treatment involves the following disadvantages:
1) a high temperature slab heating furnace for exclusive use with grain-oriented electrical steel is necessary; PA1 2) the energy raw unit of the heating furnace is high; and PA1 3) the quantity of the molten scale increases and adversely influences operations such as the scrape-out of slag. PA1 (1) control of the reduction ratio in rough hot rolling and the time between rough hot rolling and finish hot rolling; PA1 (2) control of start temperature of finish hot rolling in accordance with the quantities of acid-soluble Al and N in the slab; PA1 (3) control of the slab heating temperature, in accordance with the quantities of acid-soluble Al and N of the slab, in combination with control of start temperature of the finish hot rolling; PA1 (4) control of deviation inside the coil of start temperatures of the finish hot rolling in accordance with the quantities of acid-soluble Al and N in the slab; and PA1 (5) addition of Sn.
These problems can be avoided by reducing the slab heating temperature to the heating temperature of ordinary steel. However, this means a reduction in the quantity of MnS effective as an inhibitor or non-using of MnS, and essentially invites destabilization of the secondary recrystallization. Accordingly, in order to accomplish slab heating at low temperature, the inhibitors must be reinforced, in one way or other, by precipitates other than MnS and the normal grain growth during finish annealing must be sufficiently restricted.
In addition to the sulfides, nitrides, oxides and grain boundary segreation elements are known as such inhibitors, and the following references are known. Japanese Examined Patent Publication (Kokoku) No. 54-24685 causes grain boundary segregation elements such as As, Bi, Sn, Sb, etc., to be contained in the steel so as to lower the slab heating temperature to the range of 1,050.degree. to 1,350.degree. C., and Japanese Unexamined Patent Publication (Kokai) No. 52-24116 discloses a method which lowers the slab heating temperature to the range of 1,100.degree. to 1,260.degree. C. by causing nitride formation elements such as Zr, Ti, B, Nb, Ta, V, Cr, Mo, etc., besides Al, to be contained. Japanese Unexamined Patent Publication (Kokai) No. 57-158322 discloses a technology which carries out slab heating at low temperature by reducing the Mn content and lowering the Mn/S ratio to below 2.5, and stabilizes the secondary recrystallization by further adding Cu.
A technology which improves the structure in combination with reinforcement of these inhibitors has also been disclosed. In other words, Japanese Unexamined Patent Publication (Kokai) No. 57-89433 adds elements such as S, Se, Sb, Bi, Pb, Sn, B, etc., in addition to Mn, combines these elements with the ratio of columnar crystal in the slab and a secondary cold rolling ratio at the second cold-rolling stage so as to accomplish low temperature slab heating at 1,100.degree. to 1,250.degree. C. Further, Japanese Unexamined Patent Publication (Kokai) No. 59-190324 discloses a method which constitutes an inhibitor using Al, B and N as the primary elements in addition to S or Se, applies pulse annealing at the time of primary recrystallization annealing after cold rolling, and thus stabilizes secondary recrystallization.
As described above, great efforts have been made to accomplish slab heating at low temperature in the production of directional electromagnetic steel sheets. Further, Japanese Unexamined Patent Publication (Kokai) No. 59-56522 discloses a technology which can accomplish slab heating at low temperature by setting the amount of Mn to 0.08 to 0.45% and the amount of S to not greater than 0.007%. This method can solve the problem of the occurrence of linear secondary recrystallization defects of the product resulting from coarsening of the slab crystal grain during slab heating at high temperature.
Although the method of low-temperature slab heating is originally directed to reduce the cost of production, it cannot be put into industrial application unless it can stably produce excellent magnetic properties. To make low-temperature slab heating industrially applicable, the inventors of the present invention have developed a technique which is based on (1) control of the mean grain size of primary recrystallization before final finish annealing and (2) a nitriding treatment of the steel sheet at the intermediate stage from the completion of hot rolling to the start of secondary recrystallization at the final finish annealing. The nitride formed by this nitriding treatment mainly changes to AlN at the onset of secondary recrystallization. In other words, this method uses AlN as an inhibitor which hardly changes at a high temperature and in this sense, it is an essential condition that the slab contains
On the other hand, the presence of an excessive amount of N in the slab must be reconsidered from the component system of this technology. In other words, if essential AS and a certain amount of N are contained in the slab, AlN is formed during the process steps from slab heating to decarbonization annealing, and the AlN affects the grain growth of the primary recrystallization grain at the time of decarbonization annealing.
As a result of investigations into means for controlling the precipitation of AlN in the process steps described above, it is an object of the present invention to provide a production method for a grain oriented electrical steel sheet having excellent properties, which is free of magnetic fluctuation even when hot rolling annealing is omitted, with slab heating at low temperature.